Turbine blade

ABSTRACT

The invention relates to the design of (particularly) a fixed blade in an axial flow steam turbine. A conventional blade is of prismatic form with an aerofoil cross-section and extends radially between annular end blocks. The inventive blade is curved in the radial (height) direction, being convex from root to tip on its pressure face. The setting angle relative to the corresponding prismatic blade is therefore fine at the root and tip and coarse at the blade mid-height. The throat opening is thus reduced at root and tip and increased at mid-height so displacing some mass flow away from both lossy end wall regions. A considerable gain in the stage efficiency results. A further improvement results from providing an optimum corner fillet between the blades and end walls at the root and tip.

This is a request for filing a continuation application under 37 CFR1.62, of prior application Ser. No. 08/515,369 filed on Aug. 15, 1995,now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a turbine blade and to a turbine incorporatingthe blade. While the invention is primarily concerned with steamturbines it is also applicable to other turbines and to compressors. Theterm "turbine" is used in this specification to include machines of thiskind having aerofoil blades. It is also primarily concerned with fixedblades in turbines but is not exclusive to them.

Turbine efficiency is of great importance, particularly in largeinstallations where a fractional increase in efficiency can produce verylarge cost savings. A considerable amount of money and effort iscontinually expended therefore on research into the blade design, thisbeing a critical component.

For many years the conventional blade has been of aerofoilcross-section, the (fixed) blade extending radially between inner andouter end blocks, and the blade being of prismatic form, i.e. generatedby a line moving parallel to itself and intersecting an aerofoilsection. The orientations of both fixed and moving blades about theirrespective blade axes has also been standardised for this prismaticblade design, this orientation being defined by the blade stagger anglebetween the turbine axial direction and a line tangential to the bladeleading edge and trailing edge circles on the pressure face of theaerofoil blade.

A known improvement in the performance of the prismatic blade in theturbine is achieved by imposing a `lean` on the blade, i.e. tilting itabout its root in a circumferential plane i.e. one transverse, orperpendicular, to the turbine axis. This `lean` produces a variation inthe mass flow at outlet of the blade from the root to the tip. Theradially inner and outer ends of the blade are referred to as the rootand the tip despite the fact that both root and `tip` are terminated bythe end walls of the supporting rings 21 and 22 shown in theaccompanying FIG. 1.

Since the circumferential spacing of the blades (ie pitch) increasesprogressively from the root to tip, the position where the throat lineintersects the suction surface moves upstream with increased radius.Owing to the convex curvature of the suction surface this leads to anincrease in the outlet angle from about 13° at the root (relative to thetangential direction) to about 15° at the tip. This is illustrated inthe accompanying FIG. 6.

The blade outlet angle ∝ is illustrated in FIGS. 3(a) and 3(b) of theaccompanying drawings and is defined as sin⁻¹ (throat/blade pitch).

From the same figures the following parameters appear. The throat is theshortest width in the blade passages. It normally extends from thepressure surface of a blade at the trailing edge and is orthogonal tothe suction surface of the adjacent blade.

The stagger angle is the angle between the axis of the turbine and thetangent line touching the leading and trailing circles of the aerofoilsection.

The blade chord length is the overall extent of the blade along thestagger angle tangent line.

Modifications to the basic prismatic blade design have in the past beenproposed. For example, in the Hitachi Review Vol 27, No. 3 of 1978,twisted and other blade forms were proposed. In what was referred to asthe `controlled vortex nozzle design` there was described a nozzle (i.e.fixed blade) which conformed to the conventional prismatic blade formfor the lower half of its radial height but which had a progressivelyfiner setting angle for the upper half. The setting angle is the angleby which the aerofoil section at any blade height is rotated within itsown plane from the normal disposition for a prismatic blade. A finesetting indicates a rotation of the aerofoil section to reduce thethroat and thus reduce the outlet angle and a coarse setting a rotationto increase it. FIG. 3 of this earlier article illustrates a continuousrotation of the blade section from the root to the tip, the settingangle becoming finer with increased blade height.

Despite the fairly comprehensive analysis of blade design and settingangle of this earlier study it has been found that none of the designsinvestigated achieve the degree of improvement that the presentinvention provides.

Thus, it is an object of the present invention to provide a blade designwhich provides a significant improvement in performance over previouslyknown designs.

SUMMARY OF THE INVENTION

According to the present invention, a turbine blade for use as one of aring of similar blades arranged in the annular path of a turbine workingfluid, is of at least approximately constant aerofoil cross-section fromits root at the radially inner end to its tip at the radially outer end,and is substantially symmetrically curved between the root and the tipso that the pressure face of the aerofoil blade is convex in the radialdirection between root and tip.

The aerofoil sections at the root and tip of the blade are rotated intheir own plane relative to the mid-height section by an anglepreferably within the range 5°±2°, and preferably again, within therange 5°±1°.

The aerofoil sections of the blade preferably lie on a parabolic curvebetween the root and the tip.

The trailing edge of the blade is preferably straight from root to tip,the convex curvature of the blade pressure face in the radial directionbeing achieved by rotational displacement of the aerofoil sections aboutthe straight trailing edge.

According to an aspect of the invention, in a turbine incorporatingfixed blades as aforesaid, the ratio of the throat between adjacentfixed blades to the pitch of the blades gives the sine of an outletangle for the blades which is preferably in the range 7° to 11° at theroot of the blades, and more preferably in the range 8° to 10°.

The setting angle of the mid-height section of the fixed blades ispreferably such, in combination with the root outlet angle, as toprovide a total passage throat area equal to that of a turbine havingprismatic blades of the same stagger angle.

In a turbine having a series of stages adapted for decreasing fluiddensity of the working fluid, the outlet angle at the root of the bladesis constant throughout the series of stages and the setting angle forthe blade aerofoil sections at the radial mid-height of the blades issuch as to maintain a predetermined throat area for the blades of eachstage. This predetermined throat area is the throat area provided byprismatic blades in the corresponding stage of an otherwise similarconventional turbine.

According to a feature of the invention, the blades and their associatedend blocks are formed integrally and machined to provide fillets betweenthe blade aerofoil surfaces and the end walls. These fillets preferablyhave a radius in the range 0.15 to 0.3 of the throat dimension betweeneither the root or the tip of adjacent blades, according to where thefillets are located. Alternatively, a mean-value throat dimension may beused instead, in which case there is an advantage of greater ease ofmanufacture, since all the fillets are the same, but a disadvantage ofreduced gains in efficiency. This mean value is preferably taken as themean value of the throat dimensions between adjacent blades taken at theroot and the tip of the blades. The fillet radius more preferably liesin the range 0.2 to 0.25 of the value of throat dimension used, and inparticular approximately 0.23 of that value.

BRIEF DESCRIPTION OF THE DRAWINGS

A turbine blade in accordance with the invention, and its incorporationin a steam turbine, will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic sectional view on the axis of a steam turbineshowing a conventional `disc and diaphragm` high/intermediate pressuresteam turbine stage including an assembly of fixed blades;

FIG. 2 is a perspective view of two such conventional blades in thefixed blade diaphragm;

FIG. 3(a) is a diagrammatic radial view of the blades of FIG. 2;

FIG. 3(b) is a diagram illustrating the outlet angle from the fixedblades;

FIG. 4 is a perspective view of a fixed blade according to theinvention. The grid pattern shown on the surface is not of coursepresent in reality but serves to emphasize the curved formation of theblade;

FIG. 5 is a graph of blade section setting angle against height of thesection from root to tip of the blade, for conventional prismatic bladeand blade according to the invention;

FIG. 6 is a graph of blade outlet angle against section height, againfor the two types of blade;

FIG. 7 is a partial cross-section of the throat passage between twoblades showing the fillets formed between the two blades and the endblock, and

FIG. 8 is a diagram showing a trailing blade edge with a conventionalfillet and one with a "faired-out" fillet.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, there is shown in FIG. 1 a diagrammaticaxial section view of a conventional `disc and diaphragm`high/intermediate pressure steam turbine stage. The direction of flow Fof the working fluid, steam, is approximately parallel to the turbinerotor axis A. The rotor 10 has, for each stage, a disc 11 to which issecured a set or row of circumferentially aligned and spaced apartmoving blades 12, the blades 12 having a shroud 13 attached to theirradially outer ends. Energy in the steam flowing in the direction F fromthe front to the rear of the turbine is converted into mechanical energyin the rotor 12. For each stage, a fixed blade assembly precedes the setof moving blades 12 and is secured to the turbine inner casing 20. Thisfixed blade assembly comprises a radially inner ring 21, a radiallyouter ring 22 and a row of circumferentially aligned and spaced apartfixed blades 23, each blade 23 being secured at an inner end to theinner ring 21 and at an outer end to the outer ring 22, and each bladehaving a leading edge 24 facing the flow and a trailing edge 25. Theassembly of blades 23 with the inner and outer rings 21, 22 is known asa diaphragm. The disc and diaphragm stage as shown in FIG. 1 is of thetype in which the area between the inner and outer rings 21, 22orthogonal to the turbine axis A is larger at the fixed blade trailingedges 25 than at the blade leading edges 24. Furthermore, in the exampleshown in FIG. 1, the surfaces, i.e. the end walls, of the rings (or endblocks) 21, 22 to which the blades 23 are secured have a frusto-conicalshape diverging from the turbine axis A in the direction F from theleading (24) to the trailing (25) edges of the blades 23.

Referring now to FIG. 2, there is shown a rear view of part of a fixedblade assembly which is of the type shown in FIG. 1. The fixed blades 23shown in FIG. 2 are of the conventional prismatic kind, that is, theyare each straight, i.e. designed such that the notional aerofoilsections of the blade, each considered orthogonal to a radial line fromthe turbine axis, have the same shape from the blade inner end to theblade outer end are untwisted from the root end to the tip end and arestacked with the leading edge 24 and the trailing edge 25 each on astraight line. Each blade 23 has a concave pressure side 26 and a convexsuction side 27.

Referring to FIG. 3(a) this illustrates, in a radial plan view, theorientation of the fixed blades 23 and 29 relative to the turbine axis Aand the transverse (ie tangential or circumferential) plane T containingthe fixed blade ring and to which the axis A is perpendicular. The bladeaerofoil section is based on a small trailing edge circle 15 and alarger leading edge circle 17. The tangent line 19 to these two circlesis at an angle ψ, the stagger angle, from the axis A direction.

If a perpendicular line is drawn from the suction face 27 of blade 23 tomeet the pressure face 26 of the adjacent blade 29, and then if theshortest such line is taken, this is the throat dimension t, whichoccurs in the region of the trailing edge 25 of the blade 29. The ratioof this dimension t to the pitch p of the fixed blades gives the sine ofwhat is known as the outlet angle α. It may be seen that, approximately,this angle is the outlet angle from each blade relative to thetransverse plane T.

FIG. 4 shows a blade which is shaped in accordance with the principlesof the invention. It has a straight trailing edge 25 like theconventional prismatic blade but the remainder of the blade, and inparticular the leading edge 24, is not straight but is curved in amanner such that the pressure face of the blade is convex in the radialdirection between root and tip, that is, in a plane which is transverseto the general steam flow direction between the blades. One such plane31 is indicated in FIG. 4, the convex curvature in this plane on thepressure face 26 being obscured but conforming to that at the leadingedge 24.

More specifically this curvature is illustrated in FIG. 5 by the changein the setting angle of the various aerofoil sections 33 of FIG. 4 fromthe root 35 to the tip 37 of the blade. The individual aerofoil sections33 may be considered as being rotated in their own planes about thetrailing edge 25 by a setting angle which is positive in the centralpart of the radial height, and negative in the root and tip portions.`Positive` is taken to be a rotation toward the pressure face 26 andnegative toward the suction face 27.

In the particular example of FIG. 5, a zero setting angle occurs atabout one-fifth and four-fifths of the radial blade height where theaerofoil section has the same stagger angle, i.e. the same orientationrelative to the turbine axis, as a conventional prismatic blade in anotherwise similar conventional turbine. This `conventional` staggerangle is assumed to be 48.5°.

The setting angle varies from about minus 2.5° at the root and tip toplus 2.5° at the centre of the radial height. This is a preferredarrangement where the conventional, i.e. reference, stagger angle is48.5°. However, variations in the setting angle 5° difference will stillproduce efficiency benefits if only to a lesser extent. It is envisagedthat a variation of±2° on the 5° difference will still be beneficial,i.e. a range of setting angle differences from 3° between root/tip andcentre height, to 7° between root/tip and centre height. It is preferredhowever to limit the variation to±1° i.e. differences from 40° to 6°.

The variation of setting angle throughout the height of the blade ispreferably parabolic, as illustrated in FIG. 5.

It would to some extent be acceptable to skew the aerofoil sectionsabout some other axis than the trailing edge 25, for example the leadingedge 24 or some intermediate axis. However, the choice of the trailingedge as the rotation axis has several advantages. It keeps the criticalinterspace gap between the fixed and downstream moving blades constant.This gap has an important influence upon the unsteady aerodynamic forceson the moving blade and also on the stage efficiency via boundary layergrowth on the end walls. Secondly, by building the curvature largelyinto the leading edge a "compound lean" effect is incorporated into theleading edge area of the blade where secondary flows are generated.These secondary flows comprise vortices in parallel with the main flowthe vortices being near the end walls between adjacent fixed blades. Bythe use of the compound curved blade of the invention, over the inner(i.e. lower) half of the blade height the pressure surface pointsradially inwards, and over the outer half of the blade height thepressure surface points radially outwards. The body forces exerted onthe flow are counteracted by higher static pressures on the end walls.This results in lower velocities near the end walls and hence lowerfrictional losses.

Referring now to FIG. 6, this illustrates the relation between outletangle ∝ and radial height of the blade section (33 in FIG. 4).

In the conventional, prismatic, case, the outlet angle increases almostlinearly from about 13° at the blade root to about 15° at the tip. Thisincrease in the opening corresponds simply to the increase in the bladepitch with increasing radius. In a turbine stage incorporating the fixedblade of this embodiment, and having a form defined by the setting anglegraph of FIG. 5, the outlet angle varies from about 9.6° at the root toabout 15.6° at the mid-height and back to 12° at the tip. This asymmetrysimilarly derives from the increase in the blade pitch with radius sincethe throat moves upstream (on the suction surface) with increase ofpitch and since the throat increases faster than the pitch the outletangle increases with pitch and therefore with radius. This difference ofoutlet angle between tip and root is despite the setting angle being thesame at tip and root.

The effect of the curved blade according to the invention is to reducethe flow through both of the high loss regions near the root and tip endwalls and increase the flow through the more efficient mid-heightregion.

The best prismatic design of which the Applicants are aware is onehaving a straight negative lean of 8°, i.e., in which the fixed bladeslean in the transverse plane in a direction toward the suction face atan angle of 8° to the radius through their root. The curved blade of thepresent invention when tested in a two-stage air turbine has shown anefficiency gain of 0.8% compared to this "best" conventional design.

It is thought that a benefit arises not only in the fixed blade row butalso in the downstream moving blade row, as lower mass flows are passedinto the end wall regions where there are high secondary losses.

Where the inventive concept is applied to a series of stages in acomplete high pressure or intermediate pressure cylinder, where theblade height increases as the steam density decreases, the followingtechnique is used:

(a) the outlet angle of the blade section at the root is maintained atabout 9° throughout the stages;

(b) the same setting angle is used at the tip as for the root, i.e. theblade is symmetrical about the mid-height;

(c) the setting angle at the mid-height section is chosen to keep themean throat (over the blade height) the same as for a prismatic blade inthe same stage. This keeps the stage reaction at the same level as forthe corresponding conventional design.

(d) a parabolic distribution of setting angle over the blade height isused as for the basic design.

It may be seen that for a series of stages the blade form is simplystretched radially according to the height of the stage blade.

While the invention has been described in relation to the use of `shortheight` HP/IP fixed blades in a steam turbine of the low reaction discand diaphragm type, it is also applicable to other types of axial flowturbine and compressor, and to moving as well as fixed blades.

A further feature of the invention concerns the construction of fixedblades between their end blocks. The blades are machined or cast ingroups integrally with their end blocks which are sections of the rings21 and 22 (FIGS. 1 and 2). The blade units are then machined to providethe necessary accurate dimensioning and surface finish.

FIG. 7 is a diagram of a cross-section of part of the throat passagebetween two fixed blades. It has been found that the radius of thefillet between the end blocks 21 and 22 has a significant effect on thestage efficiency. The optimum fillet radius has been found to be in therange 0.15 to 0.3 of the throat dimension, with a preferred part of thisrange being 0.2 to 0.25 and, in particular, 0.23.

Clearly, since the throat opening at the tip is different from that atthe root (due to the increase of pitch with radius), the optimum filletradius at the outer end block will be different from that at the innerend block. Thus, the preferred, optimum values of radius are:

    .sup.r Fillet, root=0.233×opening.sub.root

    .sup.r Fillet, tip=0.233×opening.sub.tip.

However, the use of two different values of radius requires the use ofdifferent cutting tools during the manufacturing process, and it ispossible to compromise by having just one radius value which is anaverage of the above values, i.e.: ##EQU1##

Tests in a two-stage air turbine using the above "average" fillet radiusin conjunction with the described "controlled flow" blade design show astage efficiency gain of around 1.2% relative to the best conventionaldesign with prismatic fixed blades (set with -5° straight "lean" of thetrailing edges).

It is also advantageous, in order to reduce blockage effects, to "fairout" the fillet downstream of the trailing edge of the blade. This isshown in FIG. 8, where Figure 8a represents an end view of a trailingedge 25 with a conventional fillet between the edge 25 and the end wall21, and FIG. 8b represents the same view but with a "faired-out" fillet.A partial side view of FIG. 8b is shown in FIG. 8c, where the fillet canbe clearly seen to disappear to zero at its most downstream point fromthe trailing edge.

We claim:
 1. A turbine blade for use as one of a ring of similar turbineblades arranged in an annular path of a turbine working fluid, the bladebeing of at least approximately constant aerofoil cross-section from aroot at a radially inner end of the blade to a tip at a radially outerend of the blade, the blade being of substantially parabolic curvaturebetween the root and the tip so that a pressure face of the blade isconvex in a radial direction between the root and the tip, the bladehaving a trailing edge which is substantially straight from the root tothe tip, the blade comprising a plurality of aerofoil sections stackedbetween the root and the tip and lying in respective planes, the convexcurvature of the blade pressure face in the radial direction beingachieved by rotational displacement of the aerofoil sections in saidrespective planes about the substantially straight trailing edge.
 2. Theturbine blade according to claim 1, wherein ones of said aerofoilsections disposed at the root and the tip of the blade are rotated insaid respective planes about said substantially straight trailing edgerelative to a mid-height section by an angle within the range 5°±2°. 3.The turbine blade according to claim 2, wherein said ones of saidaerofoil sections are rotated by an angle within the range 5°±1°.
 4. Aturbine comprising: at least one stage having a plurality of fixedblades, each of said fixed blades being of at least approximatelyconstant aerofoil cross-section from a root at a radially inner end of arespective blade to a tip at a radially outer end of the respectiveblade, each blade being substantially symmetrically curved between theroot and the tip so that a pressure face of the blade is convex in aradial direction between the root and the tip, the blade having atrailing edge which is substantially straight from the root to the tip,the blade comprising a plurality of aerofoil sections stacked betweenthe root and the tip and lying in respective planes, the convexcurvature of the blade pressure face in the radial direction beingachieved by rotational displacement of the aerofoil sections in saidrespective planes about the substantially straight trailing edge, andadjacent fixed blades having a throat and a pitch in a ratio giving thesine of an outlet angle for the blades which is in the range 7° to 11°at the root of the blades.
 5. The turbine according to claim 4, whereinsaid outlet angle is in the range 8° to 10°0.
 6. The turbine accordingto claim 4, wherein a setting angle of the mid-height section of thefixed blades is such, in combination with the root outlet angle, as toprovide a total passage throat area equal to that of a turbine havingprismatic blades of the same stagger angle.
 7. A turbine comprising: aseries of turbine stages adapted for decreasing fluid density of aturbine working fluid, each of said stages comprising a plurality ofblades, each of said blades being of at least approximately constantaerofoil cross-section from a root at a radially inner end of arespective blade to a tip at a radially outer end of the respectiveblade, each blade being substantially symmetrically curved between theroot and the tip so that a pressure face of the blade is convex in aradial direction between the root and the tip, the blade having atrailing edge which is substantially straight from the root to the tip,the blade comprising a plurality of aerofoil sections stacked betweenthe root and the tip and lying in respective planes, the convexcurvature of the blade pressure face in the radial direction beingachieved by rotational displacement of the aerofoil sections in saidrespective planes about the substantially straight trailing edge, asetting angle at the root of the blades being constant throughout theseries of stages, and a setting angle for the aerofoil sections at amid-height of the blades being such as to maintain a predeterminedthroat area for the blades of each stage.
 8. The turbine according toclaim 7, wherein said predetermined throat area is the throat areaprovided by prismatic blades in a corresponding stage of an otherwisesimilar conventional turbine.